US 7304785 B2
The invention relates to light modulators for use in display apparatus. In particular, the invention relates to methods for supporting and support structures for light modulators to reduce unwanted motion.
1. A display apparatus, comprising:
a modulator for selectively interacting with light in an optical path to form an image on the display apparatus;
a controllable first electrostatic actuator providing a first mechanical support for the modulator, said first mechanical support providing a supportive connection from a first location on the modulator to a surface over which the modulator is supported; and
a second mechanical support providing a supportive connection from a second location on the modulator to the surface,
wherein the first electrostatic actuator drives the modulator in a plane substantially parallel to the surface.
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This application is a continuation in part of U.S. patent application Ser. No. 11/218,690, filed Sep. 2, 2005. This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 60/676,053, entitled “MEMS Based Optical Display” and filed on Apr. 29, 2005; and U.S. Provisional Patent Application No. 60/655,827, entitled MEMS Based Optical Display Modules” and filed on Feb. 23, 2005. All three applications are incorporated herein by reference
In general, the invention relates to the field of video displays, in particular, the invention relates to mechanically actuated display apparatus.
Displays built from mechanical light modulators are an attractive alternative to displays based on liquid crystal technology. Mechanical light modulators are fast enough to display video content with good viewing angles and with a wide range of color and grey scale. Mechanical light modulators have been successful in projection display applications. Backlit displays using mechanical light modulators have not yet demonstrated sufficiently attractive combinations of brightness and low power. There is a need in the art for fast, bright, low-powered mechanically actuated displays. Specifically there is a need for mechanically actuated displays that include bi-stable mechanisms and that can be driven at low voltages for reduced power consumption.
In one aspect, the invention relates to displays built from mechanical actuators which incorporate two compliant electrodes. The actuators may be controlled by passive or active matrix arrays coupling controllable voltage sources to the voltage inputs of the mechanical actuators.
The compliant electrodes in each actuator are positioned proximate to one another, such that in response to the application of a voltage across the electrodes, the electrodes are drawn together. The electrodes may be drawn together directly or progressively. At least one of the electrodes couples to a modulator which contributes to the formation of an image. According to one feature of the invention, at least a majority of the lengths of the electrodes are compliant. The electrodes may be about 0.5 μm to about 5 μm wide. In one implementation, the height of the electrodes is at least about 1.4 times the width of the electrodes. The electrodes may also be coated, at least in part with an insulator, preferably having a dielectric constant of about 1.5 or greater.
The modulator may be, for example, a shutter, a deformable mirror, a color filter, or a set of three color filters. Shutters move substantially in a plane parallel to a surface over which they are supported. The surface may have one or more apertures allowing the passage of light through the surface. The apertures may be patterned through a reflective film disposed on a substantially transparent glass or plastic substrate. If the surface has more than one aperture, the shutter includes a corresponding number of shutter apertures. By moving the shutter, the display apparatus can selectively interact with light in an optical path passing through the apertures in the surface by either blocking reflecting, absorbing, polarizing, diffracting and/or filtering the light. In various embodiments, the shutter may also be coated with a reflective or light absorbing film.
In one embodiment, one end of each electrode is anchored to the surface and the other end is free to move. In this embodiment, the modulator couples to the free end of one of the electrodes. The width of the electrode may be constant along its length, or it may vary. For example, it may become thinner closer to the modulator. Alternatively, the electrode may have thinner sections and thicker sections at multiple locations along the length of the electrode. The varying thicknesses provide for varying electrode stiffnesses. The embodiment may include an optional feature of including a third compliant electrode. The compliant electrodes not coupled to the modulator act separately as open and close drive electrodes. One or both of the drive electrodes may be curved in its natural, deactivated state. In some implementations, the drive electrodes have a first or second order curve. In other implementations, the drive electrodes have a greater than second order curve.
In another embodiment of the shutter assembly, a shutter couples to a pair of actuators at about the linear center of one side of the modulator. The actuators each include two compliant electrodes. A first compliant electrode of each actuator couples to the shutter with a spring. The first compliant electrode may also couple to the anchor with a spring. The other ends of the compliant electrodes couple to anchors, thereby connecting the shutter to two locations on a substrate. The electrodes serve as mechanical supports providing supportive connections from locations on the shutter to the substrate. A separate elastic member, such as a return spring, may couple to an opposite side of the shutter, providing an additional supportive connection for the shutter. Alternatively, a second pair of actuators may couple to the opposite side of the shutter instead of the return spring. The multiple supportive connections help reduce rotation or other movement of the shutter out of its intended plane of motion.
In a second aspect of the invention, the display apparatus includes a mechanically bi-stable shutter assembly to form an image. A mechanically bi-stable shutter assembly includes a shutter, a voltage input for receiving an actuation potential and an actuator that moves a shutter over a substrate between two mechanically stable positions. In one embodiment, the work needed to move the shutter from its first mechanically stable position to its second mechanically stable position is greater than the work need to return the shutter to its first mechanically stable position. In another embodiment, the amount of work needed to move the shutter from its first mechanically stable position to its second mechanically stable position is substantially equal to the work needed to return the shutter to its first mechanically stable position.
According to one feature of the invention, the mechanically stable positions of the shutter are provided by the state, including the position or shape, of a mechanically compliant member. In one embodiment, the mechanically compliant member is part of the actuator. In other embodiments, the mechanically compliant member is outside of the actuator. The mechanically compliant member has a first mechanically stable state in a first of the shutter's mechanically stable positions and a second mechanically stable state in the second of the shutter's mechanically stable position. Moving the shutter from the first mechanically stable position to the second mechanically stable position requires the deformation of the compliant member.
For example, the compliant member may be a curved compliant beam. When the shutter is in the first stable position, the beam has a first curvature. The beam has a second curvature when the shutter is in the second position. The curvature-may be generally “s” shaped or it may form a cosine shaped bow. In the first shutter position, the beam may bow in one direction. In transitioning to the second shutter position, the beam is deformed such that bows in an opposite direction.
Alternatively, the compliant beam may be straight while the shutter is in one of its mechanically stable position. The compliant beam forms a first angle with the shutter in the first mechanically stable shutter position. In the second position, the compliant beam forms a different angle with the shutter.
The shutter assembly may also include a second compliant member. The first and second compliant members, in one embodiment, serve as electrodes in a dual compliant beam electrode actuator. One or both beams may have two mechanically stable states. Upon application of a voltage across the compliant members, one of the compliant members deforms from one position to a second position. The voltage may result from an actuation potential being applied to one of the compliant members from one or more anchors coupled to one or both ends of the compliant member. For example, in the first position, the first compliant electrodes bows away from the second compliant electrode. In the second position, the first compliant electrode bows towards the second compliant electrode, having a substantially similar bow as the second compliant electrode. In other implementations, the first and second compliant beams form part of an thermoelectric actuator coupled to the shutter for moving it between the first and second stable positions. Regardless of the type of actuator moving the shutter, to move the shutter, a force must be applied to either the first or second compliant member.
In some embodiments, the first and second compliant member shapes are themselves mechanically stable.
The shutter assembly, in one implementation, includes a second actuator coupled to the shutter. The two actuators couple to the shutter in different locations on the shutter. According to one implementation, the actuators couple to opposite sides of the shutter, at about the middle of the sides. Compliant members in the actuators provide supportive connections for the shutter from two shutter locations to two substrate locations. According to another optional feature, at least one of the compliant members coupled to the shutter couples to two anchors, one on either end the compliant member.
In still other embodiments, the compliant members are incorporated into a stabilizer which provides the mechanical stability for the mechanically stable shutter positions. The compliant members in a stabilizer may be connected to one another. In such an embodiment, the stabilizer may provide for a third mechanically stable shutter position. The shutter is driven into the third mechanically stable shutter position in response to an application of a second actuation voltage to the voltage input. Alternatively, the compliant beams may form a stabilizer by coupling to anchors on either side of the shutter to sides of the shutter. The compliant members may include compliant or rigid beams. If the compliant members include rigid beams, the compliant members include additional compliant joints between the rigid beams to provide a degree of compliance.
Additional features of the various display apparatus include the incorporation of a working fluid among the compliant members. The working fluid preferably has a dielectric constant of at least about 1.5. The display apparatus may also include a backlight for illuminating the image.
In another aspect, the invention relates to a method of manufacturing a display apparatus. The method includes patterning a first surface to form a modulator for selectively interacting with light in an optical path. An actuator is then fabricated in the first surface connecting the modulator and an anchor. The anchor and the actuator serve as a first mechanical support, physically supporting the modulator over a second surface. The actuator is configured to drive the shutter in a plane substantially parallel to the second surface. The method further includes fabricating a second mechanical support into first surface connecting the modulator and a second anchor. The second mechanical support physically supports the modulator over the second surface. The first anchor and the second anchor are connected to two distinct locations on the second surface.
In another aspect, the invention relates to a method of forming an image. The method includes selectively applying an actuation potential to a voltage input of a shutter assembly. A shutter is moved in a plane substantially parallel to a surface, in response to the application of the actuation voltage. The shutter is moved from a first mechanically stable position to a second mechanically stable position, thereby permitting light to contribute to the formation of an image.
In still a further aspect, the invention relates to a method of forming an image on a display. The method includes selecting a light modulator and providing an actuator. The actuator includes two mechanically compliant electrodes positioned proximate to one another, at least one of which couples to a shutter. The actuator is activated by generating a voltage between the two mechanically compliant electrodes. As a result, the compliant electrodes deform as they are drawn closer together. In addition, the activation of the actuator results in movement of the shutter into or out of an optical path to affect the illumination of a pixel in the image.
The system and methods may be better understood from the following illustrative description with reference to the following drawings in which:
Each shutter assembly 102 includes a shutter 112 and an aperture 114. To illuminate a pixel 106 in the image 104, the shutter 112 is positioned such that it allows light to pass, without any significant obstruction, through, the aperture 114 towards a viewer. To keep a pixel 106 unlit, the shutter 112 is positioned such that it obstructs the passage of light through the aperture 114. The aperture 114 is defined by an opening patterned through a reflective or light-absorbing material in each shutter assembly 102.
In alternative implementations, a display apparatus 100 includes multiple shutter assemblies 102 for each pixel 106. For example, the display apparatus 100 may include three color-specific shutter assemblies 102. By selectively opening one or more of the color-specific shutter assemblies 102 corresponding to a particular pixel 106, the display apparatus 100 can generate a color pixel 106 in the image 104. In another example, the display apparatus 100 includes two or more shutter assemblies 102 per pixel 106 to provide grayscale in an image 104. In still other implementations, the display apparatus 100 may include other forms of light modulators, such as micromirrors, filters, polarizers, interferometric devices, and other suitable devices, instead of shutter assemblies 102 to modulate light to form an image.
The shutter assemblies 102 of the display apparatus 100 are formed using standard micromachining techniques known in the art, including lithography; etching techniques, such as wet chemical, dry, and photoresist removal; thermal oxidation of silicon; electroplating and electroless plating; diffusion processes, such as boron, phosphorus, arsenic, and antimony diffusion; ion implantation; film deposition, such as evaporation (filament, electron beam, flash, and shadowing and step coverage), sputtering, chemical vapor deposition (CVD), plasma enhanced CVD, epitaxy (vapor phase, liquid phase, and molecular beam), electroplating, screen printing, and lamination. See generally Jaeger, Introduction to Microelectronic Fabrication (Addison-Wesley Publishing Co., Reading Mass. 1988); Runyan, et al., Semiconductor Integrated Circuit Processing Technology (Addison-Wesley Publishing Co., Reading Mass. 1990); Proceedings of the IEEE Micro Electro Mechanical Systems Conference 1987-1998; Rai-Choudhury, ed., Handbook of Microlithography, Micromachining & Microfabrication (SPIE Optical Engineering Press, Bellingham, Wash. 1997).
More specifically, multiple layers of material (typically alternating between metals and dielectrics) are deposited on top of a substrate forming a stack. After one or more layers of material are added to the stack, patterns are applied to a top most layer of the stack marking material either to be removed from, or to remain on, the stack. Various etching techniques, including wet or dry etches or reactive ion etching, are then applied to the patterned stack to remove unwanted material. The etch process may remove material from one or more layers of the stack based on the chemistry of the etch, the layers in the stack, and the amount of time the etch is applied. The manufacturing process may include multiple iterations of layering, patterning, and etching.
In one implementation the shutter assemblies 102 are fabricated upon a transparent glass or plastic substrate. This substrate may be made an integral part of a backlight which acts to evenly distribute the illumination from lamp 105 before the light exits through apertures 114. Alternatively and optionally the transparent substrate may be placed on top of a planar light guide, wherein the array of shutter assemblies 102 act as light modulation elements in the formation of an image. In one implementation the shutter assemblies 102 are fabricated in conjunction with or subsequent to the fabrication of a thin film transistor (TFT) array on the same glass or plastic substrate. The TFT array provides a switching matrix for distribution of electrical signals to the shutter assemblies.
The process also includes a release step. To provide freedom for parts to move in the resulting device, sacrificial material is interdisposed in the stack proximate to material that will form moving parts in the completed device. An etch removes much of the sacrificial material, thereby freeing the parts to move.
After release, one or more of the surfaces of the shutter assembly may be insulated so that charge does not transfer between moving parts upon contact. This can be accomplished by thermal oxidation and/or by conformal chemical vapor deposition of an insulator such as Al2O3, Cr2O3, TiO2, TiSiO4, HfO2, HfSiO4, V2O5, Nb2O5, Ta2O5, SiO2, or Si3N4 or by depositing similar materials using techniques such as atomic layer deposition and others. The insulated surfaces are chemically passivated to prevent problems such as stiction between surfaces in contact by chemical conversion processes such as fluoridation, silanization, or hydrogenation of the insulated surfaces.
Dual compliant electrode actuators make up one suitable class of actuators for driving the shutters 112 in the shutter assemblies 102. A dual compliant beam electrode actuator, in general, is formed from two or more at least partially compliant beams. At least two of the beams serve as electrodes ( also referred to herein as “beam electrodes”). In response to applying a voltage across the beam electrodes, the beams electrodes are attracted to one another from the resultant electrostatic forces. Both beams in a dual compliant beam electrode are, at least in part, compliant. That is, at least some portion of each of the beams can flex and or bend to aid in the beams being brought together. In some implementations the compliance is achieved by the inclusion of flexures or pin joints. Some portion of the beams may be substantially rigid or fixed in place. Preferably, at least the majority of the length of the beams are compliant.
Dual compliant electrode actuators have advantages over other actuators known in the art. Electrostatic comb drives are well suited for actuating over relatively long distances, but can generate only relatively weak forces. Parallel plate or parallel beam actuators can generate relatively large forces but require small gaps between the parallel plates or beams and therefore only actuate over relatively small distances. R. Legtenberg et. al. (Journal of Microelectromechanical Systems v.6, p. 257, 1997) demonstrated how the use of curved electrode actuators can generate relatively large forces and result in relatively large displacements. The voltages required to initiate actuation in Legtenberg, however, are still substantial. As shown herein such voltages can be reduced by allowing for the movement or flexure of both electrodes.
In a dual compliant beam electrode actuator-based shutter assembly, a shutter is coupled to at least one beam of a dual compliant beam electrode actuator. As one of the beams in the actuator is pulled towards the other, the pulled beam moves the shutter, too. In doing so, the shutter is moved from a first position to a second position. In one of the positions, the shutter interacts with light in an optical path by, for example, and without limitation, blocking, reflecting, absorbing, filtering, polarizing, diffracting, or otherwise altering a property or path of the light. The shutter may be coated with a reflective or light absorbing film to improve its interferential properties. In the second position, the shutter allows the light to pass by, relatively unobstructed.
The shutter 202 a of the shutter assembly 200 a is formed from a solid, substantially planar, body. The shutter 202 a can take virtually any shape, either regular or irregular, such that in a closed position the shutter 202 a sufficiently obstructs the optical path through the aperture 206 a in the surface 204 a. In addition, the shutter 202 a must have a width consistent with the width of the aperture, that, in the open position (as depicted), sufficient light can pass through the aperture 206 a in the surface 204 a to illuminate a pixel, or contribute to the illumination of a pixel, in the display apparatus.
The shutter 202 a couples to one end of a load beam 208 a. A load anchor 210 a, at the opposite end of the load beam 208 a physically connects the load beam 208 a to the surface 204 a and electrically connects the load beam 208 a to driver circuitry in the surface 204 a. Together, the load 208 a beam and load anchor 210 a serve as a mechanical support for supporting the shutter 202 a over the surface 204 a.
The shutter assembly 200 a includes a pair of drive beams 212 a and 214 a, one located along either side of the load beam 210 a. Together, the drive beams 212 a and 214 a and the load beam 210 a form an actuator. One drive beam 212 a serves as a shutter open electrode and the other drive beam 214 a serves as a shutter close electrode. Drive anchors 216 a and 218 a located at the ends of the drive beams 212 a and 214 a closest to the shutter 202 a physically and electrically connects each drive beam 212 a and 214 a to the surface 204 a. In this embodiment, the other ends and most of the lengths of the drive beams 212 a and 214 a remain unanchored or free. The free ends of the drive beams 212 a and 214 a are closer to the anchored end of the load beam 208 a than the anchored ends of the drive beams 212 a and 214 a are to shutter end of the load beam 208 a.
The load beam 208 a and the drive beams 212 a and 214 a are compliant. That is, they have sufficient flexibility and resiliency that they can be bent out of their unstressed (“rest”) position or shape to at least some useful degree, without fatigue or fracture. As the load beam 208 a and the drive beams 212 a and 214 a are anchored only at one end, the majority of the lengths of the beams 208 a, 212 a, and 214 a is free to move, bend, flex, or deform in response to an applied force. The operation of the cantilever dual compliant beam electrode actuator based-shutter assembly 200 a is discussed further below in relation to
Changing the state of a shutter assembly that includes multiple shutter apertures with a corresponding number of surface apertures requires less shutter movement than changing the state of a shutter assembly incorporating a solid shutter and single surface aperture, while still providing for the same aperture area. Reduced required motion corresponds to lower required actuation voltage. More particularly, a decrease in required motion by ⅓ reduces the necessary actuation voltage of the actuator by a factor of about ⅓. Reduced actuation voltage further corresponds to reduced power consumption. Since the total aperture area for either shutter assembly is about the same, each shutter assembly provides a substantially similar brightness.
In other implementations, the shutter apertures and corresponding surface apertures have shapes other than slots. The apertures may be circular, polygonal or irregular. In alternative implementations, the shutter may include more shutter apertures than there are surface apertures in the shutter assembly. In such implementations, one or more of the shutter apertures may be serve as a filter, such as color filter. For example, the shutter assembly may have three shutter apertures for every surface aperture, each shutter aperture including a red, blue, or green colored filter.
As indicated in the diagram 3B, for actuators having separation functions in which n is less than or equal to 2, the application of a minimum actuation voltage (V2) results in a cascading attraction of the load beam to the drive beam without requiring the application of a higher voltage. For such actuators, the incremental increase in electrostatic force on the beams resulting from the load beam getting closer to the drive beam is greater than the incremental increase in stress on the beams needed for further displacement of the beams.
For actuators having separation functions in which x is greater than 2, the application of a particular voltage results in a distinct partial displacement of the load electrode. That is, the incremental increase in electrostatic force on the beams resulting from a particular decrease in separation between the beams, at some point, fails to exceed the incremental deformation force needed to be imparted on the load beam to continue reducing the separation. Thus, for actuators having separation functions having n greater than 2, the application of a first voltage level results in a first corresponding displacement of the load electrode. A higher voltage results in a greater corresponding displacement of the load electrode. How the shapes and relative compliance of thin beam electrodes effects actuation voltage is discussed in more detail in the following references: (R. Legtenberg et. al., Journal of Microelectromechanical Systems v.6, p. 257 (1997) and J. Li et. al. Transducers '03, The 12th International Conference on Solid State Sensors, Actuators, and Microsystems, p. 480 (2003) each of which is incorporated herein by reference
Referring back to
With reference back to
This shutter assemblies 200 a and 200 b, as well as shutter assemblies 500 (see
The reflection platform 402 may or may not be conductive. In implementations in which the reflection platform 402 is conductive, the reflection platform serves as a load electrode for the mirror-based light modulator 400. The reflection platform 402 is physically supported over, and is electrically coupled to, a substrate 404 via a compliant support member 406. If the reflection platform 402 is formed from a non-conductive material, the reflection platform 402 is coupled to a compliant conductive load beam or other form of compliant load electrode. A compliant support member 406 physically supports the, combined reflection platform 402 and electrode over the substrate 404. The support member 406 also provides an electrical connection from the electrode to the substrate 404.
The mirror-based light modulator 400 includes a second compliant electrode 408, which serves a drive electrode 408. The drive electrode 408 is supported between the substrate 404 and the reflection platform 402 by a substantially rigid second support member 410. The second support member 410 also electrically connects the second compliant electrode 408 to a voltage source for driving the mirror-based light modulator 400.
The mirror-based light modulator 400 depicted in
To form an image, light 412 is directed at an array of mirror-based light modulators 400 at a particular angle. Mirror-based light modulators 400 in their rest states reflect the light 412 away from the viewer or the display screen, and mirror-based light modulators in the active state reflect the light 412 towards a viewer or a display screen, or visa versa.
In addition, in comparison to the shutter assemblies 202 a and 202 b, the load beam 504 has a width which varies along its length. The load beam 504 is wider near its anchor 508 than it is nearer to the shutter 502. In comparison to the shutter assemblies 202 a and 202 b and because of its tailored width, the load beam 504 typically has an overall greater stiffness. Shutter assemblies incorporating stiffer beams typically require higher voltages for actuation, but in return, allow for higher switching rates. For example, the shutter assemblies 202 a and 202 b may be switched up to about 10 kHz, while the stiffer shutter assembly 500 may be switched up to about 100 kHz.
Each actuator 602 includes a compliant load member 606 connecting the shutter 604 to a load anchor 608. The compliant load members 606 each include a load beam 610 and an L bracket 612. The load anchors 608 along with the compliant load members 606 serve as mechanical supports, keeping the shutter 604 suspended proximate to the surface. The load anchors 608 physically connect the compliant load members 606 and the shutter 604 to the surface and electrically connect the load beams 610 of the load members 606 to ground. The coupling of the shutter 604 from two positions on one side of the shutter 604 to load anchors 608 in positions on either side of the shutter assembly 600 help reduce twisting motion of the shutter 604 about its central axis 614 during motion.
The L brackets 612 reduce the in-plane stiffness of the load beam. 610. That is, the L brackets 612 reduce the resistance of actuators 602 to movement in a plane parallel to the surface (referred to as “in-plane movement” 615), by relieving axial stresses in the load beam.
Each actuator 602 also includes a compliant drive beam 616 positioned adjacent to each load beam 610. The drive beams 616 couple at one end to a drive beam anchor 618 shared between the drive beams 616. The other end of each drive beam 616 is free to move. Each drive beam 616 is curved such that it is closest to the load beam 610 near the free end of the drive beam 616 and the anchored end of the load beam 610.
In operation, a display apparatus incorporating the shutter assembly 600 applies an electric potential to the drive beams 616 via the drive beam anchor 618. As a result of a potential difference between the drive beams 616 and the load beam 610, the free ends of the drive beams 616 are pulled towards the anchored ends of the load beams 610 and the shutter ends of the load beams 610 are pulled toward the anchored ends of the drive beams 616. The electrostatic force draws the shutter 604 towards the drive anchor 618. The compliant members 606 act as springs, such that when the electrical potentials are removed from the drive beams 616, the load beams compliant members 606 push the shutter 604 back into its initial position, releasing the stress stored in the load beams 610. The L brackets 612 also serve as springs, applying further restoration force to the shutter 604.
In fabrication of shutter assemblies 200 through 800, as well as for shutter assemblies 1300 through 1800 it is preferable to provide a rectangular shape for the cross section of the load beams (such as load beams 610) and the drive beams (such as drive beams 616). By providing a beam thickness (in the direction perpendicular to surface) which is 1.4 times or more larger in dimension than the beam width (in a direction parallel to the surface) the stiffness of the load beam 610 will be increased for out-of-plane motion 617 versus in-plane motion 615. Such a dimensional and, by consequence, stiffness differential helps to ensure that the motion of the shutter 604, initiated by the actuators 602, is restricted to motion along the surface and across the surface apertures as opposed to out-of-plane motion 617 which would a wasteful application of energy. It is preferable for certain applications that the cross section of the load beams (such as 610) be rectangular as opposed to curved or elliptical in shape. The strongest actuation force is achieved if the opposing beam electrodes have flat faces so that upon actuation they can approach and touch each other with the smallest possible separation distance.
In operation, to open the shutter 812, a display apparatus incorporating the shutter assembly 800 applies an electric potential to the drive anchor 822 of the shutter open actuators 802 and 804, drawing the shutter 812 towards the open position. To close the shutter 812, the display apparatus applies an electric potential to the drive anchor 822 of the shutter close actuators 806 and 808 drawing the shutter 812 towards the closed position. If neither pair of actuators 802/804 or 806/808 are activated, the shutter 812 remains in an intermediate position, somewhere between fully open and fully closed.
The shutter open actuators 802/804 and shutter closed actuators 806/808 couple to the shutter 812 at opposite ends of the shutter. The shutter open and closed actuators have their own load members 810, thus reducing the actuation voltage of each actuator 802, 804, 806 and 808. Because of the-electrical bi-stability described in reference to
For shutter assemblies such as in
Each of the shutter assemblies 200 a, 200 b, 500, 600, 700 and 800, and the mirror-based light modulator 400, described above fall into a class of light modulators referred to herein as “elastic light modulators.” Elastic light modulators have one mechanically stable rest state. In the rest state, the light modulator may be on (open or reflecting), off (closed or not reflecting), or somewhere in between (partially open or partially reflecting). If the generation of a voltage across beams in an actuator forces the light modulator out of its rest state into a mechanically unstable state, some level of voltage across the beams must be maintained for the light modulator to remain in that unstable state.
The active matrix array is fabricated as a diffused or thin-film-deposited electrical circuit on the surface of a substrate on which the elastic light modulators 902 are formed. The active matrix array 900 includes a series of row electrodes 904 and column electrodes 906 forming a grid like pattern on the substrate, dividing the substrate into a plurality of grid segments 908. The active matrix array 900 includes a set of drivers 910 and an array of non-linear electrical components, comprised of either diodes or transistors that selectively apply potentials to grid segments 908 to control one or more elastic light modulators 902 contained within the grid segments 908. The art of thin film transistor arrays is described in Active Matrix Liquid Crystal Displays: Fundamentals and Applications by Willem den Boer (Elsevier, Amsterdam, 2005).
Each grid segment 908 contributes to the illumination of a pixel, and includes one or more elastic light modulators 902. In grid segments including only a single elastic light modulator 902, the grid segment 908 includes, in addition to the elastic light modulator 902, least one diode or transistor 912 and optionally a capacitor 914. The capacitor 914 shown in
The active matrix array 900 generates an image, in one implementation by, one at a time, applying a potential from one of the drivers 910 to a selected row electrode 904, activating a corresponding row of grid segments 908. While a particular row is activated, the display apparatus applies a potential to the column electrodes corresponding to grid segments in the active row containing light modulators which need to be switched out of a rest state.
When a row is subsequently deactivated, a stored charge will remain on the electrodes of the actuator 902 (as determined by the equivalent capacitance of the actuator) as well as, optionally, on the parallel capacitor 914 that can be designed into the circuit, keeping the elastic shutter mechanisms 902 in their mechanically unstable states. The elastic shutter mechanism 902 remains in the mechanically unstable state until the voltage stored in the capacitor 914 dissipates or until the voltage is intentionally reset to ground potential during a subsequent row selection or activation step.
The active matrix array 1000 includes one row electrode 1004 for each row in the active matrix array 1000 and two column electrodes 1006 a and 1006 b for each column in the active matrix array 1000. For example, for display apparatus including shutter-based light modulators, one column electrode 1006 a for each column corresponds to the shutter open actuators of light modulators 1002 in the column. The other column electrode 1006 b corresponds to the shutter close actuators of the light modulators 1002 in the column. The active matrix array 1000 divides the substrate upon which it is deposited into grid sections 1008. Each grid section 1008 includes one or more light modulators 1002 and at least two diodes or transistors 1010 a and 1010 b and optionally two capacitors 1012 a and 1012 b. The bases 1014 a and 1014 b of each transistor 1010 a and 1010 b are electrically coupled to a column electrode 1006 a or 1006 b. The emitters 1016 a and 1016 b of the transistors 1010 a and 1010 b are coupled to a corresponding capacitor 1012 a or 1012 b and a drive electrode of the light modulator(s) 1002 in the grid section 1008.
In operation, a driver applies a potential to a selected row electrode 1004, activating the row. The active matrix array 1000 selectively applies potentials to one of the two column electrodes 1006 a or 1006 b of each column in which the state of the light modulator(s) 1002 in the grid section 1008 needs to be changed. Alternatively, the active matrix array 1000 may also apply a potential to column electrodes 1006 a or 1006 b for grid sections 1008 previously in an active state which are to remain in an active state.
For both active matrix arrays 900 and 1000, the drivers powering the column electrodes, in some implementations, select from multiple possible potentials to apply to individual column electrodes 1006 a and 1006 b. The light modulator(s) 1002 in those columns can then be opened or closed different amounts to create grayscale images.
In addition to the shutter 812, the shutter open actuators 802 and 804, the shutter close actuators 806 and 808, the load anchors 814 and the drive anchors 822, the shutter assembly 800 includes a row electrode 1104, a shutter open electrode 1106, a shutter close electrode 1108, and three surface apertures 1110. The depicted shutter assembly has at least three functional layers, which may be referred to as the row conductor layer, the column conductor layer, and the shutter layer. The shutter assembly is preferably made on a transparent substrate such as glass or plastic. Alternatively the substrate can be made from an opaque material such as silicon, as long as through holes are provided at the positions of each of the surface apertures 1110 for the transmission of light. The first metal layer on top of the substrate is the row conductor layer which is patterned into row conductor electrodes 1104 as well as reflective surface sections 1105. The reflective surface sections 1105 reflect light passing through the substrate 1102 back through the substrate 1102 except at the surface apertures 1110. In some implementations the surface apertures may include or be covered by red, green, or blue color filtering materials.
The shutter open electrode 1106 and the shutter close electrode 1108 are formed in a column conductor layer 1112 deposited on the substrate 1102, on top of the row conductor layer 1104. The column conductor layer 1112 is separated from the row conductor layer 1104 by one or more intervening layers of dielectric material or metal. The shutter open electrode 1104 and the shutter close electrode 1106 of the shutter assembly 800 are shared with other shutter assemblies in the same column of the display apparatus. The column conductor layer 1112 also serves to reflect light passing through gaps in the ground electrode 1104 other than through the surface apertures 1110. The row conductor layer 1104 and the column conductor layer 1112 are between about 0.1 and about 2 microns thick. In alternative implementations, the column conductor 1112 layer can be located below the row conductor layer 1104. In another alternative implementation both the column conductor layer and the row conductor layer may be located above the shutter layer.
The shutter 812, the shutter open actuators 802 and 804, the shutter close actuators 806 and 808, the load anchors 814 and the drive anchors 822 are formed from third functional layer of the shutter assembly 800, referred to as the shutter layer 1114. The actuators 802, 804, 806, and 808 are formed from a deposited metal, such as, without limitation, Au, Cr or Ni, or a deposited semiconductor, such as, without limitation as polycrystalline silicon, or amorphous silicon, or from single crystal silicon if formed on top of a buried oxide (also known as silicon on insulator). The beams of the actuators 802, 804, 806, and 808 are patterned to dimensions of about 0.2 to about 20 microns in width. The shutter thickness is typically in the range of 0.5 microns to 10 microns. To promote the in-plane movement of the shutters (i.e. reduce the transverse beam stiffness as opposed to the out-of-plane stiffness), it is preferable to maintain a beam dimensional ratio of about at least 1.4:1, with the beams being thicker than they are wide.
Metal or semiconductor vias electrically connect the row electrode 1104 and the shutter open electrode 1106 and the shutter close electrode 1108 of the column conductor layer 1112 to features on the shutter layer 1114. Specifically, vias 1116 electrically couple the row electrode 1104 to the load anchors 814 of the shutter assembly 800, keeping the compliant load member 810 of the shutter open actuators 802 and 804 and the shutter close actuators 806 and 808, as well as the shutter 812, at the row conductor potential. Additional vias electrically couple the shutter open electrode 1106 to the drive beams 820 of the shutter open actuators 802 and 804 via the drive anchor 822 shared by the shutter open actuators 802 and 804. Still other vias electrically couple the shutter close electrode 1108 to the drive beams 820 of the of the shutter close actuators 806 and 808 via the drive anchor 822 shared by the shutter close actuators 806 and 808.
The shutter layer 1114 is separated from the column conductor layer 1112 by a lubricant, vacuum or air, providing the shutter 812 freedom of movement. The moving pieces in the shutter layer 1114 are mechanically separated from neighboring components (except their anchor points 814) in a release step, which can be a chemical etch or ashing process, which removes a sacrificial material from between all moving parts.
The diodes, transistors, and/or capacitors (not shown for purpose of clarity) employed in the active matrix array may be patterned into the existing structure of the three functional layers, or they can be built into separate layers that are disposed either between the shutter assembly and the substrate or on top of the shutter layer. The reflective surface sections 1105 may be patterned as extensions of the row and column conductor electrodes or they can be patterned as free-standing or electrically floating sections of reflective material. Alternatively the reflective surface sections 1105 along with their associated surface apertures 1110 can be patterned into a fourth functional layer, disposed between the shutter assembly and the substrate, and formed from either a deposited metal layer or a dielectric mirror. Grounding conductors may be added separately from the row conductor electrodes in layer 1104. These separate grounding conductors may be required when the rows are activated through transistors, such as is the case with an active matrix array. The grounding conductors can be either laid out in parallel with the row electrodes (and bussed together in the drive circuits), or the grounding electrodes can be placed into separate layers between the shutter assembly and the substrate.
In addition to elastic light modulators, display apparatus can include bi-stable light modulators, for example bi-stable shutter assemblies. As described above, a shutter in an elastic shutter assembly has one mechanically stable position (the “rest position”), with all other shutter positions being mechanically unstable. The shutter of a bi-stable shutter assembly, on the other hand, has two mechanically stable positions, for example, open and closed. Mechanically bi-stable shutter assemblies have the advantage that no voltage is required to maintain the shutters in either the open or the closed positions. Bi-stable shutter assemblies can be further subdivided into two classes: shutter assemblies in which each stable position is substantially energetically equal, and shutter assemblies in which one stable position is energetically preferential to the other mechanically stable position.
Each dual compliant electrode actuator 1304 and 1306 includes a compliant member 1310 connecting the shutter 1302, at about its linear axis 1312, to two load anchors 1314, located in the corners of the shutter layer 1300. The compliant members 1310 each include a conductive load beam 1316, which may have an insulator disposed on part of, or the entirety of its surface. The load beams 1316 server as mechanical supports, physically supporting the shutter 1302 over a substrate on which the shutter assembly is built. The actuators 1304 and 1306 also each include two compliant drive beams 1318 extending from a shared drive anchor 1320. Each drive anchor 1320 physically and electrically connects the drive beams 1318 to the substrate. The drive beams 1318 of the actuators 1304 and 1306 curve away from their corresponding drive anchors 1320 towards the points on the load anchors 1314 at which load beams 1316 couple to the load anchors 1314. These curves in the drive beams 1318 act to reduce the stiffness of the drive beams, thereby helping to decrease the actuation voltage.
Each load beam 1316 is generally curved, for example in a bowed (or sinusoidal) shape. The extent of the bow is determined by the relative distance between the load anchors 1314 and the length of the load beam 1316. The curvatures of the load beams 1316 provide the bi-stability for the shutter assembly 1300. As the load beam 1316 is compliant, the load beam 1316 can either bow towards or away from the drive anchor 1320. The direction of the bow changes depending on what position the shutter 1302 is in. As depicted, the shutter 1302 is in the closed position. The load beam 1316 of the shutter open actuator 1304 bows away from the drive anchor 1320 of the shutter open actuator 1304. The load beam 1316 of the shutter closed actuator 1306 bows towards the drive anchor 1320 of the shutter close actuator 1306.
In operation, to change states, for example from closed to open, a display apparatus applies a potential to the drive beams 1318 of the shutter open actuator 1304. The display apparatus may also apply an potential to the load beams 1316 of the shutter open actuator. Any electrical potential difference between the drive beams and the load beams, regardless of sign with respect to a ground potential, will generate an electrostatic force between the beams. The resultant voltage between the drive beams 1318 and the load beams 1316 of the shutter open actuator 1304 results in an electrostatic force, drawing the beams 1316 and 1318 together. If the voltage is sufficiently strong, the load beam 1316 deforms until its curvature is substantially reversed, as depicted in the shutter close actuator in
For load beams fabricated in silicon, typical as-designed widths are about 0.2 μm to about 10 μm. Typical as-designed lengths are about 20 μm to about 1000 μm. Typical as-designed beam thicknesses are about 0.2 μm to about 10 μm. The amount by which the load beam is pre-bent is typically greater than three times the as-designed width
The load beams of
Another design configuration for
The pre-stress condition of the shutter assembly can be provided by a number of means. The condition can be imposed post-manufacture by, for instance, mechanically packaging the substrate to induce a substrate curvature and thus a surface strain in the system. A pre-stressed condition can also be imposed as a thin film stress imposed by surface layers on or around the load beams. These thin film stresses result from the particulars of a deposition processes. Deposition parameters that can impart a thin film stress include thin film material composition, deposition rate, and ion bombardment rate during the deposition process.
In addition, the widths of the load beams 1402 vary along their lengths. In particular, the beams are narrower along sections where they meet the load anchors 1404 and the shutter 1406. The points along the load beams 1402 at which the load beams 1402 become wider serve as pivot points 1410 to confine deformation of the load beams 1402 to the narrower sections 1410.
The shutter open actuator 1502, by itself, is an elastic actuator, having one mechanically stable state. Unless otherwise constrained, the shutter open actuator 1502, after actuation would return to its rest state. The shutter open actuator 1502 includes two load beams 1510 coupled to load anchors 1512 by L brackets 1514 at one end and to the shutter 1516 via L brackets 1518 at the other end. In the rest state of the shutter open actuator 1502, the load beams 1510 are straight. The L brackets 1514 and 1518 allow the load beams 1510 to deform towards the drive beams 1506 of the shutter open actuator 1502 upon actuation of the shutter open actuator 1502 and away form the drive beams 1506 upon actuation of the shutter close actuator 1504.
The shutter close actuator 1504 is similarly inherently elastic. The shutter close actuator 1504 includes a single load beam 1520 coupled to a load anchor 1522 at one end. When not under stress, i.e., in its rest state, the load beam 1520 is straight. At the opposite end of the load beam 1520 of the shutter close actuator 1504, the load beam 1520 is coupled to a stabilizer 1524 formed from two curved compliant beams 1526 connected at their ends and at the center of their lengths. The beams 1526 of the stabilizer 1524 have two mechanically stable positions: bowed away from the shutter close actuator 1504 (as depicted) and bowed towards the shutter close actuator 1504.
In operation, if either the shutter open actuator 1502 or the shutter close actuator are activated 1504, the load beam 1520 of the shutter close actuator 1504 is deformed to bow towards the shutter open actuator 1504 or towards the drive beams 1528 of the shutter close actuator 1504, respectively, as the shutter 1516 is moved into an actuated position. In either case, the length of the shutter close actuator 1504 load beam 1520 with respect to the width of the shutter layer 1500 as a whole, is reduced, pulling the beams 1526 of the stabilizer 1524 to bow towards the shutter close actuator 1504. After the activated actuator is deactivated, the energy needed to deform the beams 1526 of the stabilizer 1524 back to its original position is greater than the energy stored in the load beams 1510 and 1520 and of the actuators 1502 and 1504. Additional energy must be added to the system to return the shutter 1516 to its rest position. Thus, the shutter 1516 in the shutter assembly has three mechanically stable positions, open, half open, and closed.
Unlike the bi-stable shutter assemblies described above, instead of relying upon a particular curvature of one or more beams to provide mechanical stability, the bi-stable actuator 1600 relies on the straight relaxed state of its support beams 1604 to provide mechanical stability. For example, in its two mechanically stable positions, depicted in
The shutter 1602 is coupled to two positions on either side of the shutter 1602 through support beams 1604 to anchors 1603 in positions on either side of the shutter assembly 1600, thereby reducing any twisting or rotational motion of the shutter 1602 about its central axis. The use of compliant support beams 1604 connected to separate anchors on opposite sides of the shutter 1602 also constrains the movement of the shutter along a linear translational axis. In another implementation, a pair of substantially parallel compliant support beams 1604 can be coupled to each side of shutter 1602. Each of the four support beams couples at independent and opposing points on the shutter 1602. This parallelogram approach to support of the shutter 1602 helps to guarantee that linear translational motion of the shutter is possible.
The shape of the compliant element 1718 is such that a relatively easy in-plane translation of the shutter 1704 is allowed while out-of-plane motion of the shutter is restricted.
The actuation of the bi-stable shutter assembly 1700 is accomplished by a pair of elastic dual compliant beam electrode actuators 1706, similar to the actuators employed in
Each actuator 1706 comprises a compliant load member 1720. One end of the compliant load member 1720 is coupled to the shutter 1704, while the other end is free. In shutter assembly 1700 the compliant load members in actuators 1706 a are not coupled to anchors or otherwise connected to the surface 1712. The drive beams 1722 of the actuators 1706 are coupled to anchors 1724 and thereby connected to the surface 1712. In this fashion the voltage of actuation is reduced.
The shutter 1702 b in shutter assembly 1700 b has a plurality of shutter apertures 1708 b, each of possesses a segmented shape designed to make maximum use of the rotational motion of the shutter.
Each thermoelectric actuator 1802 and 1804 includes three compliant beams 1810, 1812, and 1814. Compliant beams 1810 and 1812 are each thinner than compliant beam 1814. Each of the beams 1810, 1812, and 1814 is curved in an s-like shape, holding the shutter 1806 stably in position.
In operation, to change the position of the shutter from open (as depicted) to closed, current is passed through a circuit including beams 1810 and 1814. The thinner beams 1810 in each actuator 1802 and 1804 heat, and therefore also expands, faster than the thicker beam 1814. The expansion forces the beams 1810, 1812, and 1814 from their mechanically stable curvature, resulting in transverse motion of the shutter 1806 to the closed position. To open the shutter 1806, current is run through a circuit including beams 1812 and 1814, resulting in a similar disproportionate heating and expansion of beams 1812, resulting in the shutter 1806 being forced back to the open position.
Bi-stable shutter assemblies can be driven using a passive matrix array or an active matrix array.
The shutter assemblies 1300, 1400, 1500, 1600, 1700 a, and 1800 are amenable to the use of a passive matrix array because their property of mechanical bi-stability makes it possible to switch between open and closed states if the voltage across the actuator exceeds a minimum threshold voltage. If the drivers 1910 are programmed such that none of them will output a voltage that by itself is sufficient to switch the shutter assemblies between open and closed states, then a given shutter assembly will be switched if its actuator receives voltages from two opposing drivers 1910. The shutter assembly at the intersection of a particular row and column can be switched if it receives voltages from its particular row and column drivers whose difference exceeds the minimum threshold voltage.
To change the state of a shutter assembly 1902 from a closed state to an open state, i.e., to open the shutter assembly 1902, a driver 1910 applies a potential to the row electrode 1908 corresponding to the row of the grid in which the shutter assembly 1902 is located. A second driver 1910 applies a second potential, in some cases having an opposite polarity, to the shutter open electrode 1912 corresponding to the column in the grid in which the shutter assembly 1902 is located. To change the state of a shutter assembly 1902 from an open state to a closed state, i.e., to close the shutter assembly 1902, a driver 1910 applies a potential to the row electrode 1908 corresponding to the row of the display apparatus in which the shutter assembly 1902 is located. A second driver 1910 applies a second potential, in some cases having an opposite polarity, to the shutter close electrode 1914 corresponding to the column in the display apparatus in which the shutter assembly 1902 is located. In one implementation, a shutter assembly 1902 changes state in response to the difference in potential applied to the row electrode 1908 and one of the column electrodes 1912 or 1914 exceeding a predetermined switching threshold.
To form an image, in one implementation, a display apparatus sets the state of the shutter assemblies 1902 in the grid, one row at a time in sequential order. For a given row, the display apparatus first closes each shutter assembly 1902 in the row by applying a potential to the corresponding row electrodes 1908 and a pulse of potential to all of the shutter close electrodes 1914. Then, the display apparatus opens the shutter assemblies 1902 through which light is to pass by applying a potential to the shutter open electrode 1912 and applying a potential to the row electrodes 1908 for the rows which include shutter assemblies 1902 in the row which are to be opened. In one alternative mode of operation, instead of closing each row of shutter assemblies 1902 sequentially, after all rows in the display apparatus are set to the proper position to form an image, the display apparatus globally resets all shutter assemblies 1902 at the same time by applying a potentials to all shutter close electrodes 1914 and all row electrodes 1908 concurrently. In another alternative mode of operation, the display apparatus forgoes resetting the shutter assemblies 1902 and only alters the states of shutter assemblies 1902 that need to change state to display a subsequent image. A number of alternate driver control schemes for images have been proposed for use with ferroelectric liquid crystal displays, many of which can be incorporated for use with the mechanically bi-stable displays herein. These technologies are described in Liquid Crystal Displays: Driving Schemes and Electro-Optical Effects, Ernst Lieder (Wiley, New York, 2001).
The physical layout of the display is often a compromise between the characteristics of resolution, aperture area, and driving voltage. Small pixel sizes are generally sought to increase the resolution of the display. As pixels become smaller, however, proportionally the room available for shutter apertures decreases. Designers seek to maximize aperture ratio as this increases the brightness and power efficiency of the display. Additionally, the combination of a small pixels and large aperture ratios implies large angular deformations in the compliant members that support the shutters, which tends to increase the drive voltages required and the energy dissipated by the switching circuitry.
The tiling or pixel arrangements for shutter assemblies need not be limited to the constraints of a square array. Dense tiling can also be achieved using rectangular, rhombehedral, or hexagonal arrays of pixels, all of which find applications, for example in video and color imaging displays.
The epoxy seal 2128 seals in a working fluid 2130. The working fluid 2130 is engineered with viscosities preferably below about 10 centipoise and with relative dielectric constant preferably above about 2.0, and dielectric breakdown strengths above about 104 V/cm. The working fluid 2130 can also serve as a lubricant. Its mechanical and electrical properties are also effective at reducing the voltage necessary for moving the shutter between open and closed positions. In one implementation, the working fluid 2130 preferably has a low refractive index, preferably less than about 1.5. In another implementation the working fluid 2130 has a refractive index that matches that of the substrate 2104. Suitable working fluids 2130 include, without limitation, de-ionized water, methanol, ethanol, silicone oils, fluorinated silicone oils, dimethylsiloxane, polydimethylsiloxane, hexamethyldisiloxane, and diethylbenzene.
A sheet metal or molded plastic assembly bracket 2132 holds the cover plate 2122, shutter assemblies 2102, the substrate 2104, the backlight 2116 and the other component parts together around the edges. The assembly bracket 2132 is fastened with screws or indent tabs to add rigidity to the combined display apparatus 2100. In some implementations, the light source 2118 is molded in place by an epoxy potting compound.